| Experimental factors | Coded level | ||
| −1 | 0 | +1 | |
| Actual level | |||
| Concentration of activating agent: X1 (mol·L−1) | 1 | 3 | 5 |
| Carbonization temperature: X2 (˚C) | 400 | 550 | 700 |
| Carbonization time: X3 (min) | 30 | 75 | 120 |